The present invention relates to monitoring storage batteries during charging. More specifically, the present invention relates to detection of a thermal runaway condition during charging of storage batteries.
Thermal runaway is becoming an increasingly important concern in the battery industry. Thermal runaway may be one of the most predominant concerns for many users with valve regulated lead acid (VRLA) batteries which are commonly used in telecommunications, electric utilities, railroad signals and communications, uninterruptible power systems, emergency power and military systems. Unlike a common flooded battery, VRLA batteries are much less tolerant to abusive conditions. However, frequently little consideration is given to the VRLA battery's physical arrangement, ventilation, cooling and charging characteristics.
There has been a significant increase in thermal runaway incidents in recent years, for example in the telecommunications industry. The result has been decreased reliability of telecommunication service, loss of equipment from fire or explosion, danger to persons and premature loss of capacity of the batteries which ultimately requires costly replacement. Significant consideration has been given to temperature feedback control of the charging system voltage and measurement of float currents. Such a technique is described in U.S. Pat. No. 5,214,370, issued May 25, 1993, to Harm et al and entitled BATTERY CHARGER WITH THERMAL RUNAWAY PROTECTION. However, these techniques have not provided a monitoring system of sufficient accuracy to control thermal runaway in actual applications.
Unlike a flooded battery, VRLA batteries are sealed, and access to the interior of the battery for temperature measurements is impossible. Therefore, temperature measurements have been limited to external temperature sensing which introduces a thermal lag time. This lag time is too long to accurately control charging in the event of a thermal runaway condition. An additional problem is simply deciding where to position an external temperature sensor because many of the VRLA batteries used in industrial applications are packaged in multiple cell monoblock containers.
Another technique to detect thermal runaway is to monitor the ambient temperature in close proximity to the battery under charge. Another technique includes monitoring both ambient temperature and the external temperature of the battery to detect thermal runaway. However, these techniques do little to overcome the thermal lag problem described above.
U.S. Pat. No. 4,114,083, issued Sep. 12, 1978, to Benham et al, entitled BATTERY THERMAL RUNAWAY MONITOR, describes another technique which attempts to prevent thermal runaway. The Benham et al reference describes monitoring a nickel-cadmium battery which is being charged by a constant 24 volt source in an aircraft. The system monitors charging current. Upon detection of an increase in charging current over time, the system predicts that the battery has entered a thermal runaway condition such that charging can be halted. However, this system suffers from a number of drawbacks. For example, charging current may increase with time due to conditions other than thermal runaway, such as when the battery is discharged and is capable of accepting a high charging rate. Additionally, the added charging current due to thermal runaway in a single battery in a series of batteries is relatively small and difficult to detect.
From the foregoing discussion, it can be seen that the art lacks an accurate technique which is capable of predicting the onset of thermal runaway and responsibly controlling the battery charger.